Amplifiers used in portable audio devices are typically powered by a power source providing a supply voltage Vdd. An analogue signal representing audio information is input to the amplifier. The amplifier outputs an amplified signal biased midway between the supply voltage Vdd and ground; that is, biased at Vdd/2. In this way, the amplified signal can extend across the full range of available voltages, or from the supply voltage Vdd to ground, without distortion. Moreover, this approach also ensures that both positive and negative fluctuations in the input analogue signal are represented in the amplified signal.
The amplified signal is output to a speaker, which in the case of a portable audio device is typically contained in headphones. The speaker is in turn connected to ground. As such, the bias voltage of the amplified signal results in an average voltage across the speaker of Vdd/2. As a result, there is a Direct Current (DC) component in the amplified signal that constantly passes through the speaker. This is undesirable as it may damage the speaker.
In order to prevent the DC component reaching the speaker it has been proposed to couple a capacitor in line between the output of the amplifier and the speaker. The capacitor effectively acts as a high pass filter, preventing signals below a certain frequency being propagated to the speaker. The cut-off frequency of such a filter depends on the capacitance of the capacitor, and also upon the impedance of the speaker. In practice, a large capacitance is required if all audible frequencies are to reach the headphones. In a particular example, this capacitance is 220 μF.
Capacitors having capacitance in the order of hundreds of μF typically have relatively large physical dimensions, and cannot be integrated into a microchip. Instead, the amplifier might be integrated onto the microchip and the large capacitor provided external to the microchip on a circuit board on which the microchip is mounted. In the context of a portable audio device, this limits miniaturisation and adds complexity to the device.
It has also been proposed to provide two equal and opposite supply voltages, Vdd and −Vdd, to the amplifier. The signal output by the amplifier may then be biased at ground. As a result, there is no DC component in the output signal.
An example of such a system is described in international (PCT) patent publication no. WO 2006/031304. In particular, this document describes the use of a DC voltage-to-voltage converter to provide a negative supply voltage, −Vdd, from the initial positive supply voltage, Vdd, provided by the power source.
However, the DC voltage-to-voltage converter requires additional capacitors and/or inductors which cannot be integrated onto a microchip. In a particular example, two capacitors are required, each having a capacitance of 1 μF. Although this is significantly lower than the capacitance of the in-line capacitor described above, the capacitors still cannot be integrated easily onto a microchip so must be provided external to the microchip like the in-line capacitor. Specifically, the addition of these two capacitors requires three extra pins on the microchip; one capacitor is connected between a pin and ground, while the other is connected between two separate pins. This approach therefore takes up additional space and does not miniaturise the amplifier to the extent that may be desired.
Moreover, the regulation of the DC voltage-to-voltage converter in order to provide the correct output is not trivial and must be carefully managed. As a result, there are cost implications in the application of this approach, and also potential instability in the amplifier if the regulation is unsuccessful.